One of the most important parameters required in understanding of the
diffusion phenomenon in micro fluidics is the estimation of the diffusion
coefficient or diffusion constant of the solute particles in a solvent. The
diffusion coefficient determines how fast the solute molecules move a certain
distance in the solvent. In this respect an approximate but very useful result
can be obtained from Einstein's relation for calculation of diffusion
coefficient of Brownian type motion of a spherical body. This is used in the
estimation of diffusion coefficient. There could be an error of upto 30%.

The plot shows the diffusion coefficient of spherical bodies ranging from 0.1Å
to 50,000Å in radius in water at room temperature (27°C).

Assumptions

-The solution is not highly concentrated. -The
fluid has constant viscosity. -The
viscosity of water is assumed to be 1 mPa-s.